In a mask process for forming a desired pattern in the course of manufacturing a semiconductor, patterning is performed through an exposure process and a developing process after a photosensitive film is coated on an etching target film. To suppress reflection of light during the exposure, an antireflection coating (ARC) layer is formed on the etching target film under the photosensitive film. The ARC layer may be an organic ARC layer or an inorganic ARC layer. By way of example, Patent Document 1 or 2 discloses a method for forming a pattern in an organic ARC layer by using an etching gas while suppressing reflection of light.
Since the organic ARC layer and the inorganic ARC layer have different film properties, etching gases suitable for them may be different. Thus, if an etching gas used in the etching of the organic ARC layer in Patent Document 1 or 2 is used to etch an inorganic ARC layer, effective etching may not be implemented.    Patent Document 1: Japanese Patent Laid-open Publication No. 2009-152586    Patent Document 2: Pamphlet of International Patent Publication No. WO98/32162
Especially, among inorganic ARC layers, a silicon-containing inorganic ARC layer (hereinafter, simply referred to as a “Si-ARC layer”) has been etched by using a gaseous mixture of a tetrafluoromethane (CF4) gas and an oxygen (O2) gas as an etching gas while using an ArF resist film as a mask. In the etching process by this gaseous mixture, however, since selectivity for the Si-ARC layer against the ArF resist film is not high, a great amount of the ArF resist film is etched, resulting in a problem in a subsequent process. Further, there has occurred a pattern deformation such as line edge roughness (LER) or a pattern edge deformation such as line width roughness (LWR) due to non-uniformity in line widths of a pattern.